distinct() function (not select qualifier) in postgres

Question

I just came across a SQL query, specifically against a Postgres database, that uses a function named "distinct". Namely:

select distinct(pattern) as pattern, style, ... etc ...
from styleview
where ... etc ...

Note this is NOT the ordinary DISTINCT qualifier on a SELECT -- at least it's not the normal syntax for the DISTINCT qualifier, note the parentheses. It is apparently using DISTINCT as a function, or maybe this is some special syntax.

Any idea what this means?

I tried playing with it a little and if I write

select distinct(foo)
from bar

I get the same results as

select distinct foo
from bar

When I combine it with other fields in the same select, it's not clear to me exactly what it's doing.

I can't find anything in the Postgres documentation.

Thanks for any help!

Answer 1

From the documentation:

If DISTINCT is specified, all duplicate rows are removed from the result set (one row is kept from each group of duplicates). ALL specifies the opposite: all rows are kept; that is the default.

DISTINCT ON ( expression [, ...] ) keeps only the first row of each set of rows where the given expressions evaluate to equal. The DISTINCT ON expressions are interpreted using the same rules as for ORDER BY (see above). Note that the "first row" of each set is unpredictable unless ORDER BY is used to ensure that the desired row appears first. For example,

The ON portion is optional, so it really comes down to:

1. The brackets being used
2. Placement in the query - SQL Server & MySQL throw an error if you use DISTINCT in any but the first position of the SELECT clause

PostgreSQL is the only database to my knowledge to support this syntax.